For each thread, the kernel maintains two attributes (addresses)
called set_child_tid and clear_child_tid. These two attributes
contain the value NULL by default.
set_child_tid
If a thread is started using clone(2) with the
CLONE_CHILD_SETTID flag, set_child_tid is set to the value
passed in the ctid argument of that system call.
When set_child_tid is set, the very first thing the new thread
does is to write its thread ID at this address.
clear_child_tid
If a thread is started using clone(2) with the
CLONE_CHILD_CLEARTID flag, clear_child_tid is set to the value
passed in the ctid argument of that system call.
The system call set_tid_address() sets the clear_child_tid value for
the calling thread to tidptr.
When a thread whose clear_child_tid is not NULL terminates, then, if
the thread is sharing memory with other threads, then 0 is written at
the address specified in clear_child_tid and the kernel performs the
following operation:
futex(clear_child_tid, FUTEX_WAKE, 1, NULL, NULL, 0);
The effect of this operation is to wake a single thread that is
performing a futex wait on the memory location. Errors from the
futex wake operation are ignored.

This page is part of release 4.16 of the Linux man-pages project. A
description of the project, information about reporting bugs, and the
latest version of this page, can be found at
https://www.kernel.org/doc/man-pages/.
Linux 2017-09-15 SET_TID_ADDRESS(2)